1. Field of the Invention
The present invention relates generally to a process for injection molding metallic alloys and, more particularly, to a process for injection molding semi-solid alloys having a high content of solid material.
2. Related Art
Semi-solid metals processing began as a casting process developed in the early 1970s at the Massachusetts Institute of Technology. Since then, the field of semi-solid processing has expanded to include semi-solid forging and semi-solid molding. Semi-solid processing provides a number of advantages over conventional metals-processing techniques that require the use of molten metals. One advantage is the energy savings of not having to heat metals to their melting points and maintain the metals in their molten state during processing. Another advantage is the reduced amount of liquid-metal corrosion caused by processing fully molten metals.
Semi-solid injection molding (SSIM) is a metals-processing technique that utilizes a single machine for injecting alloys in a semi-solid state into a mold to form an article of a nearly net (final) shape. In addition to the advantages of semi-solid processing mentioned above, the benefits of SSIM also include an increased design flexibility of the final article, a low-porosity article as molded (i.e., without subsequent heat treatment), a uniform article microstructure, and articles with mechanical and surface-finish properties that are superior to those made by conventional casting. Also, because the entire process takes place in one machine, alloy oxidation can be nearly eliminated. By providing an ambient environment of inert gas (e.g., argon), the formation of unwanted oxides during processing is prevented and, in turn, the recycling of scrap pieces is facilitated.
The major benefits of SSIM are primarily attributed to the presence of solid particles within the slurry of alloy material to be injection molded. The solid particles are generally believed to promote a laminar flow-front during injection molding, which minimizes porosity in the molded article. The material is partially melted by heating to temperatures between the liquidus and the solidus of the alloy being processed (the liquidus being the temperature above which the alloy is completely liquid and the solidus being the temperature below which the alloy is completely solid). SSIM avoids the formation of dendritic features in the microstructure of the molded alloy, which are generally believed to be detrimental to the mechanical properties of the molded article.
According to known SSIM processes, the percentage of solids is limited to between 0.05 to 0.60. The upper limit of 60% was determined based on a belief that any higher solids content would result in a degradation in processing yield and an inferior product. It is also generally believed that the need to prevent premature solidification during injection imposes an upper limit on the solids content of 60%.
Although a 5-60% solids content is generally understood to be the working range for SSIM, it is also generally understood that practical guidelines recommend a range of 5-10% solids for injection molding thin-walled articles (i.e., articles with fine features) and 25-30% for articles with thick walls. Moreover, it is also generally believed that, for solids contents above 30%, a post-molding solution heat-treatment is required to increase the mechanical strength of the molded article to acceptable levels. Thus, although the solids content of conventional SSIM processes generally has been accepted to be limited to 60% or lower, in practice the solids content is usually kept to 30% or lower.